Gravitational Waves Might Test For Extra Dimensions

The detection of gravitational waves last year was possible by combining state-of-the-art technology with an exquisite understanding of the theory behind it. That allowed physicists to pick a signal among the noise. Many other researchers are now interested in using this to test the limits of our physics.

The latest study suggests that gravitational waves could be used to test whether or not more than four dimensions exist. The idea, a staple of theories like string theory, suggests that the cosmos has more than the noticeable four dimensions of space-time and that gravity is so weak (compared to the other forces) because it can escape into the other dimensions.

So why not use gravitational waves to test this? David Andriot and Gustavo Lucena Gómez, from the Max Planck Institute for Gravitational Physics in Germany, have written a research paper (currently in press so yet to be peer-reviewed) about what subtle signatures the gravitational waves might show if extra dimensions do exists.

“If there are extra dimensions in the universe, then gravitational waves can walk along any dimension, even the extra dimensions,” Lucena Gómez told New Scientist.

He added: “If extra dimensions are in our universe, this would stretch or shrink space-time in a different way that standard gravitational waves would never do.”

The researchers think that the extra dimension would create two measurable effects in gravitational waves. The first one is related to excess gravity. If extra dimensions are really taking in so much gravity, we should be able to see that. According to their hypothesis, this would look like an extra signal at a higher frequency, located after the regular distribution.

The second clue is related to the properties of space-time itself, if indeed the universe has 10 or 11 dimensions. The gravitational waves would be slightly deformed as they propagate through the universe, stretched in some directions and compressed in others.

According to the researchers, the first signal would be unmistakable evidence for an extra-dimension. Nothing else could produce it. But unfortunately, detectors that could detect those frequencies are currently beyond what we have and will have in the near future.

The second signature, which they call a breathing mode, might instead be tested sooner rather than later. On top of the three active gravitational wave observatories, there are several more planned to be operational before 2030. By combining their detections, scientists could potentially see if a breathing mode is present in gravitational waves.

However, a breathing mode is not exclusive to theories with extra dimensions and could be a hint of something else. Although very much speculative, it’s interesting to see people working on how to use gravitational waves. We might not find dark matter or extra dimensions using them, but it’s better to be prepared.